Minimum Shift Keying (MSK) is a known binary modulation that impresses binary information bits onto a radio frequency carrier by rotating the phase smoothly through either +90.degree. of -90.degree. from its previous value according to the polarity of the information bit being transmitted. Thus, the phase nominally lies at 0 to 180.degree.at the end of even bits and 90 or -90 .degree. at the end of odd bits. With suitable precoding, it may be arranged that an even bit B(2i) is represented always by a terminal phase of 0.degree. for a `1` and 180 .degree.for a `0`, and that odd bits B(2i+1) are represented by 90.degree. for a `1` and -90.degree. for a `0`.
In MSK, the phase rotates smoothly at a constant rate in either a clockwise or anticlockwise direction. The constant rate of change of phase represents either a positive frequency offset or a negative frequency offset from the nominal radio carrier frequency. The frequency offset changes abruptly when the data polarity changes the direction of phase rotation.
In a known variant of MSK, called Gaussian Filtered Minimum Shift Keying (GMSK), a Gaussian filter is used to smooth the frequency transitions so that the phase rotation does not exhibit abrupt changes of direction. This smoothing effect by the Gaussian filter reduces the spectral energy in neighboring radio frequency channels and improves adjacent channel interference characteristics, at the expense of rounding the data transitions so that after a data `1` or `0` the phase may not quite reach the expected end points, with a consequent slight loss of noise immunity. A Gaussian filter was found expirically in the past to reduce adjacent channel energy the most for a given amount of rounding loss. The GMSK modulation technique is used in the European GSM cellular phone system.
GMSK is a constant amplitude modulation where the signal only varies in phase. Better spectral containment may be obtained by a so-called linear modulation where the amplitude is permitted to vary. The spectral efficiency may be measured in bits per second per Hertz of transmission bandwidth. The spectral efficiency may be increased by using quaternary modulation instead of binary modulation. For example, two bits at a time may be combined to form quaternary symbols with a value of 0, 1, 2, or 3, which are conveyed by transmitting a signal phase of 0, 90, 270, or 180.degree., respectively. Such a modulation is called Quadrature Phase Shift Keying (QPSK). Alternatively, the four phases, also known as constellation points, may be systematically shifted through 45 degrees between successive symbol periods so that even symbols are represented by 0, 90, 270, or 180.degree. while odd symbols are represented by 45, 135, -135, or -45.degree.. This alternative modulation is called Pi/4-QPSK. When in Pi/4-QPSK, the data symbol is represented by the change in phase from the previous value to the next value, being one of the four rotations +/-45 or +/-135.degree., it is known as Differential Pi/4-QPSK or Pi/4-DQPSK. QPSK, Pi/4-QPSK and Pi/4-DQPSK may all be regarded as time varying vectors in the two-dimensional complex plane that have a time varying real coordinate (I) and a time varying imaginary coordinate (Q). If the I and Q waveforms are separately linearly filtered, the spectral containment can be as good as the filter characteristics can be made but at the expense of introducing amplitude modulation, which is harder to transmit than pure phase modulation. Nevertheless, I-Q filtered Pi/4-DQPSK is the modulation used in the US digital cellular system IS-54. The IS-54 modulation achieves 1.62 bits per second per Hertz of channel bandwidth while GSM achieves 1.35 bits per second per Hertz.
Yet another modulation uses the four phase shifts +/-45 and +/-135.degree. to represent a quaternary symbol (bit pair), but smoothes the phase changes in the same way as GMSK to provide spectral containment without introducing amplitude modulation. The technique, called 4-ary CPM, is neither as spectrally efficient nor power efficient as Pi/4-DQPSK, however.
The current invention differs from all of the above in first producing two binary-modulated, constant envelope GMSK signals using a first half A(1), A(2). . . A(n) of the total number of information bits to be conveyed for the first GMSK signal and a second half B(1), B(2). . . B(n) of the total information bits to modulate the second GMSK signal.